diff --git a/beacon_node/operation_pool/src/attestation.rs b/beacon_node/operation_pool/src/attestation.rs
index 11537e6ec3b..2f7fba45401 100644
--- a/beacon_node/operation_pool/src/attestation.rs
+++ b/beacon_node/operation_pool/src/attestation.rs
@@ -90,6 +90,8 @@ impl<'a, T: EthSpec> AttMaxCover<'a, T> {
         let att_participation_flags =
             get_attestation_participation_flag_indices(state, &att.data, inclusion_delay, spec)
                 .ok()?;
+        let base_reward_per_increment =
+            altair::BaseRewardPerIncrement::new(total_active_balance, spec).ok()?;
 
         let fresh_validators_rewards = attesting_indices
             .iter()
@@ -98,7 +100,7 @@ impl<'a, T: EthSpec> AttMaxCover<'a, T> {
                 let participation = participation_list.get(index)?;
 
                 let base_reward =
-                    altair::get_base_reward(state, index, total_active_balance, spec).ok()?;
+                    altair::get_base_reward(state, index, base_reward_per_increment, spec).ok()?;
 
                 for (flag_index, weight) in PARTICIPATION_FLAG_WEIGHTS.iter().enumerate() {
                     if att_participation_flags.contains(&flag_index)
diff --git a/consensus/state_processing/src/common/altair.rs b/consensus/state_processing/src/common/altair.rs
index 6cf80bdd9ed..8943ef2f40b 100644
--- a/consensus/state_processing/src/common/altair.rs
+++ b/consensus/state_processing/src/common/altair.rs
@@ -2,27 +2,45 @@ use integer_sqrt::IntegerSquareRoot;
 use safe_arith::{ArithError, SafeArith};
 use types::*;
 
+/// This type exists to avoid confusing `total_active_balance` with `base_reward_per_increment`,
+/// since they are used in close proximity and the same type (`u64`).
+#[derive(Copy, Clone)]
+pub struct BaseRewardPerIncrement(u64);
+
+impl BaseRewardPerIncrement {
+    pub fn new(total_active_balance: u64, spec: &ChainSpec) -> Result<Self, ArithError> {
+        get_base_reward_per_increment(total_active_balance, spec).map(Self)
+    }
+
+    pub fn as_u64(&self) -> u64 {
+        self.0
+    }
+}
+
 /// Returns the base reward for some validator.
 ///
+/// The function has a different interface to the spec since it accepts the
+/// `base_reward_per_increment` without computing it each time. Avoiding the re computation has
+/// shown to be a significant optimisation.
+///
 /// Spec v1.1.0
 pub fn get_base_reward<T: EthSpec>(
     state: &BeaconState<T>,
     index: usize,
-    // Should be == get_total_active_balance(state, spec)
-    total_active_balance: u64,
+    base_reward_per_increment: BaseRewardPerIncrement,
     spec: &ChainSpec,
 ) -> Result<u64, Error> {
     state
         .get_effective_balance(index)?
         .safe_div(spec.effective_balance_increment)?
-        .safe_mul(get_base_reward_per_increment(total_active_balance, spec)?)
+        .safe_mul(base_reward_per_increment.as_u64())
         .map_err(Into::into)
 }
 
 /// Returns the base reward for some validator.
 ///
 /// Spec v1.1.0
-pub fn get_base_reward_per_increment(
+fn get_base_reward_per_increment(
     total_active_balance: u64,
     spec: &ChainSpec,
 ) -> Result<u64, ArithError> {
diff --git a/consensus/state_processing/src/per_block_processing/altair/sync_committee.rs b/consensus/state_processing/src/per_block_processing/altair/sync_committee.rs
index 8358003e4b4..306e86714c6 100644
--- a/consensus/state_processing/src/per_block_processing/altair/sync_committee.rs
+++ b/consensus/state_processing/src/per_block_processing/altair/sync_committee.rs
@@ -1,4 +1,4 @@
-use crate::common::{altair::get_base_reward_per_increment, decrease_balance, increase_balance};
+use crate::common::{altair::BaseRewardPerIncrement, decrease_balance, increase_balance};
 use crate::per_block_processing::errors::{BlockProcessingError, SyncAggregateInvalid};
 use crate::{signature_sets::sync_aggregate_signature_set, VerifySignatures};
 use safe_arith::SafeArith;
@@ -72,7 +72,8 @@ pub fn compute_sync_aggregate_rewards<T: EthSpec>(
     let total_active_balance = state.get_total_active_balance()?;
     let total_active_increments =
         total_active_balance.safe_div(spec.effective_balance_increment)?;
-    let total_base_rewards = get_base_reward_per_increment(total_active_balance, spec)?
+    let total_base_rewards = BaseRewardPerIncrement::new(total_active_balance, spec)?
+        .as_u64()
         .safe_mul(total_active_increments)?;
     let max_participant_rewards = total_base_rewards
         .safe_mul(SYNC_REWARD_WEIGHT)?
diff --git a/consensus/state_processing/src/per_block_processing/process_operations.rs b/consensus/state_processing/src/per_block_processing/process_operations.rs
index 3bf22d004a4..31a4ac1fb42 100644
--- a/consensus/state_processing/src/per_block_processing/process_operations.rs
+++ b/consensus/state_processing/src/per_block_processing/process_operations.rs
@@ -1,7 +1,8 @@
 use super::*;
 use crate::common::{
-    altair::get_base_reward, get_attestation_participation_flag_indices, increase_balance,
-    initiate_validator_exit, slash_validator,
+    altair::{get_base_reward, BaseRewardPerIncrement},
+    get_attestation_participation_flag_indices, increase_balance, initiate_validator_exit,
+    slash_validator,
 };
 use crate::per_block_processing::errors::{BlockProcessingError, IntoWithIndex};
 use crate::VerifySignatures;
@@ -128,6 +129,7 @@ pub mod altair {
 
         // Update epoch participation flags.
         let total_active_balance = state.get_total_active_balance()?;
+        let base_reward_per_increment = BaseRewardPerIncrement::new(total_active_balance, spec)?;
         let mut proposer_reward_numerator = 0;
         for index in &indexed_attestation.attesting_indices {
             let index = *index as usize;
@@ -143,7 +145,7 @@ pub mod altair {
                 {
                     validator_participation.add_flag(flag_index)?;
                     proposer_reward_numerator.safe_add_assign(
-                        get_base_reward(state, index, total_active_balance, spec)?
+                        get_base_reward(state, index, base_reward_per_increment, spec)?
                             .safe_mul(weight)?,
                     )?;
                 }
diff --git a/consensus/state_processing/src/per_epoch_processing/altair/inactivity_updates.rs b/consensus/state_processing/src/per_epoch_processing/altair/inactivity_updates.rs
index 038fe770440..967f642e85d 100644
--- a/consensus/state_processing/src/per_epoch_processing/altair/inactivity_updates.rs
+++ b/consensus/state_processing/src/per_epoch_processing/altair/inactivity_updates.rs
@@ -14,6 +14,7 @@ pub fn process_inactivity_updates<T: EthSpec>(
     participation_cache: &ParticipationCache,
     spec: &ChainSpec,
 ) -> Result<(), EpochProcessingError> {
+    let previous_epoch = state.previous_epoch();
     // Score updates based on previous epoch participation, skip genesis epoch
     if state.current_epoch() == T::genesis_epoch() {
         return Ok(());
@@ -33,7 +34,7 @@ pub fn process_inactivity_updates<T: EthSpec>(
                 .safe_add_assign(spec.inactivity_score_bias)?;
         }
         // Decrease the score of all validators for forgiveness when not during a leak
-        if !state.is_in_inactivity_leak(spec) {
+        if !state.is_in_inactivity_leak(previous_epoch, spec) {
             let inactivity_score = state.get_inactivity_score_mut(index)?;
             inactivity_score
                 .safe_sub_assign(min(spec.inactivity_score_recovery_rate, *inactivity_score))?;
diff --git a/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs b/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
index 503dadfc708..004726923e9 100644
--- a/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
+++ b/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
@@ -12,7 +12,6 @@
 //! to get useful summaries about the validator participation in an epoch.
 
 use safe_arith::{ArithError, SafeArith};
-use std::collections::HashMap;
 use types::{
     consts::altair::{
         NUM_FLAG_INDICES, TIMELY_HEAD_FLAG_INDEX, TIMELY_SOURCE_FLAG_INDEX,
@@ -24,6 +23,7 @@ use types::{
 #[derive(Debug, PartialEq)]
 pub enum Error {
     InvalidFlagIndex(usize),
+    InvalidValidatorIndex(usize),
 }
 
 /// A balance which will never be below the specified `minimum`.
@@ -64,7 +64,7 @@ struct SingleEpochParticipationCache {
     /// It would be ideal to maintain a reference to the `BeaconState` here rather than copying the
     /// `ParticipationFlags`, however that would cause us to run into mutable reference limitations
     /// upstream.
-    unslashed_participating_indices: HashMap<usize, ParticipationFlags>,
+    unslashed_participating_indices: Vec<Option<ParticipationFlags>>,
     /// Stores the sum of the balances for all validators in `self.unslashed_participating_indices`
     /// for all flags in `NUM_FLAG_INDICES`.
     ///
@@ -76,11 +76,12 @@ struct SingleEpochParticipationCache {
 }
 
 impl SingleEpochParticipationCache {
-    fn new(hashmap_len: usize, spec: &ChainSpec) -> Self {
+    fn new<T: EthSpec>(state: &BeaconState<T>, spec: &ChainSpec) -> Self {
+        let num_validators = state.validators().len();
         let zero_balance = Balance::zero(spec.effective_balance_increment);
 
         Self {
-            unslashed_participating_indices: HashMap::with_capacity(hashmap_len),
+            unslashed_participating_indices: vec![None; num_validators],
             total_flag_balances: [zero_balance; NUM_FLAG_INDICES],
             total_active_balance: zero_balance,
         }
@@ -100,7 +101,11 @@ impl SingleEpochParticipationCache {
     ///
     /// May return an error if `flag_index` is out-of-bounds.
     fn has_flag(&self, val_index: usize, flag_index: usize) -> Result<bool, Error> {
-        if let Some(participation_flags) = self.unslashed_participating_indices.get(&val_index) {
+        let participation_flags = self
+            .unslashed_participating_indices
+            .get(val_index)
+            .ok_or(Error::InvalidValidatorIndex(val_index))?;
+        if let Some(participation_flags) = participation_flags {
             participation_flags
                 .has_flag(flag_index)
                 .map_err(|_| Error::InvalidFlagIndex(flag_index))
@@ -121,13 +126,14 @@ impl SingleEpochParticipationCache {
         &mut self,
         val_index: usize,
         state: &BeaconState<T>,
+        current_epoch: Epoch,
         relative_epoch: RelativeEpoch,
     ) -> Result<(), BeaconStateError> {
         let val_balance = state.get_effective_balance(val_index)?;
         let validator = state.get_validator(val_index)?;
 
         // Sanity check to ensure the validator is active.
-        let epoch = relative_epoch.into_epoch(state.current_epoch());
+        let epoch = relative_epoch.into_epoch(current_epoch);
         if !validator.is_active_at(epoch) {
             return Err(BeaconStateError::ValidatorIsInactive { val_index });
         }
@@ -149,8 +155,10 @@ impl SingleEpochParticipationCache {
         }
 
         // Add their `ParticipationFlags` to the map.
-        self.unslashed_participating_indices
-            .insert(val_index, *epoch_participation);
+        *self
+            .unslashed_participating_indices
+            .get_mut(val_index)
+            .ok_or(BeaconStateError::UnknownValidator(val_index))? = Some(*epoch_participation);
 
         // Iterate through all the flags and increment the total flag balances for whichever flags
         // are set for `val_index`.
@@ -190,19 +198,10 @@ impl ParticipationCache {
         let current_epoch = state.current_epoch();
         let previous_epoch = state.previous_epoch();
 
-        let num_previous_epoch_active_vals = state
-            .get_cached_active_validator_indices(RelativeEpoch::Previous)?
-            .len();
-        let num_current_epoch_active_vals = state
-            .get_cached_active_validator_indices(RelativeEpoch::Current)?
-            .len();
-
         // Both the current/previous epoch participations are set to a capacity that is slightly
         // larger than required. The difference will be due slashed-but-active validators.
-        let mut current_epoch_participation =
-            SingleEpochParticipationCache::new(num_current_epoch_active_vals, spec);
-        let mut previous_epoch_participation =
-            SingleEpochParticipationCache::new(num_previous_epoch_active_vals, spec);
+        let mut current_epoch_participation = SingleEpochParticipationCache::new(state, spec);
+        let mut previous_epoch_participation = SingleEpochParticipationCache::new(state, spec);
         // Contains the set of validators which are either:
         //
         // - Active in the previous epoch.
@@ -224,6 +223,7 @@ impl ParticipationCache {
                 current_epoch_participation.process_active_validator(
                     val_index,
                     state,
+                    current_epoch,
                     RelativeEpoch::Current,
                 )?;
             }
@@ -232,13 +232,14 @@ impl ParticipationCache {
                 previous_epoch_participation.process_active_validator(
                     val_index,
                     state,
+                    current_epoch,
                     RelativeEpoch::Previous,
                 )?;
             }
 
             // Note: a validator might still be "eligible" whilst returning `false` to
             // `Validator::is_active_at`.
-            if state.is_eligible_validator(val_index)? {
+            if state.is_eligible_validator(previous_epoch, val_index)? {
                 eligible_indices.push(val_index)
             }
         }
@@ -313,16 +314,20 @@ impl ParticipationCache {
      * Active/Unslashed
      */
 
-    pub fn is_active_unslashed_in_previous_epoch(&self, val_index: usize) -> bool {
+    /// Returns `None` for an unknown `val_index`.
+    pub fn is_active_unslashed_in_previous_epoch(&self, val_index: usize) -> Option<bool> {
         self.previous_epoch_participation
             .unslashed_participating_indices
-            .contains_key(&val_index)
+            .get(val_index)
+            .map(|flags| flags.is_some())
     }
 
-    pub fn is_active_unslashed_in_current_epoch(&self, val_index: usize) -> bool {
+    /// Returns `None` for an unknown `val_index`.
+    pub fn is_active_unslashed_in_current_epoch(&self, val_index: usize) -> Option<bool> {
         self.current_epoch_participation
             .unslashed_participating_indices
-            .contains_key(&val_index)
+            .get(val_index)
+            .map(|flags| flags.is_some())
     }
 
     /*
diff --git a/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs b/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
index ce102694f58..ccebbcb3a29 100644
--- a/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
+++ b/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
@@ -6,7 +6,10 @@ use types::consts::altair::{
 };
 use types::{BeaconState, ChainSpec, EthSpec};
 
-use crate::common::{altair::get_base_reward, decrease_balance, increase_balance};
+use crate::common::{
+    altair::{get_base_reward, BaseRewardPerIncrement},
+    decrease_balance, increase_balance,
+};
 use crate::per_epoch_processing::{Delta, Error};
 
 /// Apply attester and proposer rewards.
@@ -67,13 +70,14 @@ pub fn get_flag_index_deltas<T: EthSpec>(
     let unslashed_participating_increments =
         unslashed_participating_balance.safe_div(spec.effective_balance_increment)?;
     let active_increments = total_active_balance.safe_div(spec.effective_balance_increment)?;
+    let base_reward_per_increment = BaseRewardPerIncrement::new(total_active_balance, spec)?;
 
     for &index in participation_cache.eligible_validator_indices() {
-        let base_reward = get_base_reward(state, index, total_active_balance, spec)?;
+        let base_reward = get_base_reward(state, index, base_reward_per_increment, spec)?;
         let mut delta = Delta::default();
 
         if unslashed_participating_indices.contains(index as usize)? {
-            if !state.is_in_inactivity_leak(spec) {
+            if !state.is_in_inactivity_leak(previous_epoch, spec) {
                 let reward_numerator = base_reward
                     .safe_mul(weight)?
                     .safe_mul(unslashed_participating_increments)?;
diff --git a/consensus/state_processing/src/per_epoch_processing/base/rewards_and_penalties.rs b/consensus/state_processing/src/per_epoch_processing/base/rewards_and_penalties.rs
index 99d08a6db33..87e4261e0a1 100644
--- a/consensus/state_processing/src/per_epoch_processing/base/rewards_and_penalties.rs
+++ b/consensus/state_processing/src/per_epoch_processing/base/rewards_and_penalties.rs
@@ -78,6 +78,7 @@ pub fn get_attestation_deltas<T: EthSpec>(
     validator_statuses: &ValidatorStatuses,
     spec: &ChainSpec,
 ) -> Result<Vec<AttestationDelta>, Error> {
+    let previous_epoch = state.previous_epoch();
     let finality_delay = state
         .previous_epoch()
         .safe_sub(state.finalized_checkpoint().epoch)?
@@ -92,7 +93,7 @@ pub fn get_attestation_deltas<T: EthSpec>(
         // `get_inclusion_delay_deltas`. It's safe to do so here because any validator that is in
         // the unslashed indices of the matching source attestations is active, and therefore
         // eligible.
-        if !state.is_eligible_validator(index)? {
+        if !state.is_eligible_validator(previous_epoch, index)? {
             continue;
         }
 
diff --git a/consensus/state_processing/src/per_epoch_processing/epoch_processing_summary.rs b/consensus/state_processing/src/per_epoch_processing/epoch_processing_summary.rs
index 81487474236..5e15aa3e1b2 100644
--- a/consensus/state_processing/src/per_epoch_processing/epoch_processing_summary.rs
+++ b/consensus/state_processing/src/per_epoch_processing/epoch_processing_summary.rs
@@ -101,7 +101,9 @@ impl<T: EthSpec> EpochProcessingSummary<T> {
             EpochProcessingSummary::Altair {
                 participation_cache,
                 ..
-            } => participation_cache.is_active_unslashed_in_current_epoch(val_index),
+            } => participation_cache
+                .is_active_unslashed_in_current_epoch(val_index)
+                .unwrap_or(false),
         }
     }
 
@@ -197,7 +199,9 @@ impl<T: EthSpec> EpochProcessingSummary<T> {
             EpochProcessingSummary::Altair {
                 participation_cache,
                 ..
-            } => participation_cache.is_active_unslashed_in_previous_epoch(val_index),
+            } => participation_cache
+                .is_active_unslashed_in_previous_epoch(val_index)
+                .unwrap_or(false),
         }
     }
 
diff --git a/consensus/types/src/beacon_state.rs b/consensus/types/src/beacon_state.rs
index d182ab9ae77..3a0f7d02e80 100644
--- a/consensus/types/src/beacon_state.rs
+++ b/consensus/types/src/beacon_state.rs
@@ -1602,17 +1602,23 @@ impl<T: EthSpec> BeaconState<T> {
         self.clone_with(CloneConfig::committee_caches_only())
     }
 
-    pub fn is_eligible_validator(&self, val_index: usize) -> Result<bool, Error> {
-        let previous_epoch = self.previous_epoch();
+    /// Passing `previous_epoch` to this function rather than computing it internally provides
+    /// a tangible speed improvement in state processing.
+    pub fn is_eligible_validator(
+        &self,
+        previous_epoch: Epoch,
+        val_index: usize,
+    ) -> Result<bool, Error> {
         self.get_validator(val_index).map(|val| {
             val.is_active_at(previous_epoch)
                 || (val.slashed && previous_epoch + Epoch::new(1) < val.withdrawable_epoch)
         })
     }
 
-    pub fn is_in_inactivity_leak(&self, spec: &ChainSpec) -> bool {
-        (self.previous_epoch() - self.finalized_checkpoint().epoch)
-            > spec.min_epochs_to_inactivity_penalty
+    /// Passing `previous_epoch` to this function rather than computing it internally provides
+    /// a tangible speed improvement in state processing.
+    pub fn is_in_inactivity_leak(&self, previous_epoch: Epoch, spec: &ChainSpec) -> bool {
+        (previous_epoch - self.finalized_checkpoint().epoch) > spec.min_epochs_to_inactivity_penalty
     }
 
     /// Get the `SyncCommittee` associated with the next slot. Useful because sync committees